1. Field of the Invention
The present invention relates to an analog-to-digital (A/D) conversion controlling device for converting digital voltage to analog voltage in an image forming apparatus.
2. Description of the Related Art
A fixing roller used in an electrophotographic image forming apparatus is heated to a predetermined temperature by a fixing heater employing a halogen heater or the like that generates heat by power supplied from an external power source (a commercial power supply). When an input power source can afford to supply sufficient power depending on its power usage, for example, in an idle state, the input power source charges a rechargeable auxiliary power source, such as an electric double layer capacitor. When required power is not sufficiently supplied from the input power source alone, the fixing heater is heated by the power charged in the auxiliary power source, so that the heating of the fixing roller is supported.
In view of energy saving, a system has been developed that stops the operation performed by a fixing heater that consumes high power in an idle state, and that restores the operation by drawing power from both an external power source and an auxiliary power source, so as to heat (warm up) the fixing roller in a short time. When the fixing roller is heated in a short time, an ambient temperature of the fixing roller and a temperature of a pressure roller that presses a recording medium are not sufficiently increased. As a result, when an image is formed immediately after the warming-up, the heat of the fixing roller is dissipated to the recording medium and the pressure roller, causing quick temperature drop in the fixing roller. In particular, when the ambient temperature is low for long hours, the temperatures of the recording medium and the pressure roller become low, causing the temperature drop more likely. When the voltage at the external power source is low, a low voltage is applied to the fixing heater. As a result, the heater generates less heat, causing the temperature drop in the fixing roller more likely.
Such temperature drop in the fixing roller during image formation immediately after the warming-up following the idle state has been addressed using a system that aids to heat the fixing roller by operating a fixing heater that draws power from an auxiliary power source also when the temperature drop occurs in the fixing roller. To address both the warming-up and the temperature drop in the fixing roller with all required conditions, the auxiliary power source needs to have a larger capacitance to supply more power. However, this poses problems with increased charging time, power, and cost, and with a limitation on the mountable size.
Assume that, due to the limitation on the mountable size of the auxiliary power source, the auxiliary power source is controlled based on the temperature of the fixing roller, and used to heat the fixing roller during the warming-up. In this case, the warming-up takes time, for example, when voltage drop occurs in input source voltage or in a low temperature environment. This causes the auxiliary power source to burn up an increased amount of power. As a result, sufficient power may not be left for addressing the temperature drop in the fixing roller during image formation immediately after the warming-up.
To solve these problems, a method has been proposed that includes detecting voltages of an external power source and an auxiliary power source, an ambient temperature, temperatures of a fixing roller and a pressure roller, and the like; determining, for restoration, an optimal distribution of usage power of the auxiliary power source between usage for warming up the fixing roller in a short time and usage for addressing the temperature drop in the fixing roller during image formation; and controlling the fixing heater (see FIGS. 1 and 2 described later).
In general, an analog-to-digital (A/D) converter built in or connected to a central processing unit (CPU) for controlling an image forming apparatus is used as a method for detecting analog signals of the voltages of the external power source and the auxiliary power source, the ambient temperature, the temperatures of the fixing roller and the pressure roller, and the like. Such analog signals need to be detected with high accuracy to perform accurate control. In particular, an input source voltage needs to be detected with accuracy of ±1 volt with respect to an input source voltage of alternating current (AC) 100 volts. When using an A/D converter or the like that is built in the CPU and having relatively poor accuracy, it is necessary to correct and reduce a conversion error that occurs during A/D conversion.
Conventional image forming apparatuses use an A/D converter built in a controlling CPU or connected to an external device, as a method for detecting analog signals corresponding to voltages of an input power source (a commercial alternating current source) and of an auxiliary power source, an ambient temperature, temperatures of a fixing roller and a pressure roller, and the like. Such analog signals need to be detected with high accuracy to perform accurate control. In particular, an input source voltage needs to be detected with high accuracy of +1 volt with respect to an input source voltage ranging from AC 90 volts to AC 110 volts. Due to an error in a detection circuit that converts an alternating current voltage to a direct current voltage acceptable by the A/D converter, some method is necessary to reduce conversion errors that occur during A/D conversion in the subsequent stage.
Specifically, because the A/D converter has conversion errors (such as an offset error, a full scale error, and a nonlinearity error) that occur during conversion, the conversion errors are contained in digital values. The offset error is an analog input voltage that is converted to a minimum digital value (e.g., 0) by the A/D conversion at the A/D converter. The full scale error is an analog input value that is converted to a maximum digital value (FS) by A/D conversion at the A/D converter. The nonlinearity error is caused because a digital value of an analog voltage input to the A/D converter varies in a non-linear manner. For example, with a 10-bit A/D converter causing a conversion error of 10 least significant bits (LSB), a converted value obtained by converting an input voltage includes 10/1023 (about 1%) conversion error. Further, an error in a reference source of the A/D converter is added to the converted value. In general, a high-performance A/D converter generating small conversion errors is combined with a high-accuracy reference source, as a method for detecting an analog signal with high accuracy. However, this method is more expensive than the method using the A/D converter built in or connected to the controlling CPU.
As described, with the method using the A/D converter built in or connected to the CPU for controlling the image forming apparatus to detect an analog signal, the A/D converter has poor conversion accuracy.
As a conventional technology for reducing the fluctuation after the A/D conversion, for example, Japanese Patent Application Laid-open No. 2005-26830 discloses an A/D converter that performs A/D conversion by switching a plurality of input ports. The A/D converter produces a high accuracy voltage fluctuating in a range smaller than that of an A/D conversion reference voltage. Based on a digital value obtained by A/D converting the high-accuracy voltage, correction is made on a digital value obtained by A/D converting a voltage input via another input port.
A conventional A/D conversion controlling device will now be described in detail with reference to FIGS. 11 and 12. FIGS. 11 and 12 are schematic diagrams of controllers of a conventional image forming apparatus, in which FIG. 11 is a schematic of a controller using a typical A/D converter, and FIG. 12 is a schematic of a controller using the A/D converter disclosed in Japanese Patent Application Laid-open No. 2005-26830.
In FIG. 11, a controller 1100 includes a one-chip microcomputer 1101 including a CPU 1102, a read only memory (ROM) 1103, a random access memory (RAM) 1104, an A/D converting unit 1105, an input switching unit 1106, an A/D conversion controlling unit 1107, and an I/O controlling unit 1109; and a reference voltage generator 1108 that generates a voltage. The ROM 1103 is a read-only storage device that stores therein a basic processing program implemented by the controller 1100, programs for controlling units such as a fixing unit and a scanner provided in the conventional image forming apparatus, and data necessary for implementing these programs. The RAM 1104 is a storage device capable of temporarily storing therein the data necessary for implementing the programs.
A power source 1111 drives the one-chip microcomputer 1101 of the controller 1100, and also provides an input to be used as a reference voltage to the reference voltage generator 1108. The voltage generated at the reference voltage generator 1108 is input to the A/D converting unit 1105 and used as an A/D conversion reference voltage.
The controller 1100 is connected, for example, to a sensor 1110 that generates analog signals for detection of the temperature and the voltage of each unit to control fixing, and to a fixing heater driving circuit 1112 connected to the one-chip microcomputer 1101 via the I/O controlling unit 1109 and turning ON/OFF a fixing heater for heating a fixing roller.
The A/D converting unit 1105 can convert a plurality of analog signal inputs to digital data one by one, by switching them in a time division manner with the input switching unit 1106 constituted by a semiconductor switch such as an analog switch. Such input switching performed by the input switching unit 1106 is controlled by the A/D conversion controlling unit 1107 in synchronism with the operation performed by the A/D converting unit 1105.
In contrast to the controller 1100 shown in FIG. 11, FIG. 12 is a schematic diagram of a controller 1200 of an image forming apparatus that employs a method for correcting the conversion errors in the A/D converter, disclosed as a correction of A/D conversion errors in Japanese Patent Application Laid-open No. 2005-26830. The controller 1200 has substantially the same configuration as the controller 1100 shown in FIG. 11, and only components being different from those of the controller 1100 are described. A power source 1202 drives the one-chip microcomputer 1101 of the controller 1200, and also provides an input to a reference voltage generator 1201. Further, the power source 1202 provides an input to be used as an A/D conversion reference voltage to the A/D converting unit 1105. The voltage (the reference voltage) generated at the reference voltage generator 1201 is input to the A/D converting unit 1105 via the input switching unit 1106 and used for correcting conversion errors in the A/D converting unit 1105.
FIG. 13 is a graph of conversion characteristics regarding conversion errors in a typical A/D converter. In general, as a method for calculating an analog input voltage from a digital value obtained by conversion at the A/D converting unit 1105 shown in FIG. 11, an analog input voltage is calculated from a digital value based on the A/D ideal conversion characteristics shown in FIG. 13. This method performs calculation, provided that an analog input voltage is 0 volts for a minimum digital value (0), and that an analog input voltage for a maximum digital value (FS) is equal to a reference voltage (Vref) of the A/D converting unit 1105.
As to an analog input voltage V calculated from a digital value D, its converted value V′ is calculated based on the ideal A/D conversion characteristics shown in FIG. 13, using Equation 1:V′=Vref/FS×D where V is an analog input voltage (an input from the sensor 1110 serving as a temperature sensor, a voltage sensor, or the like) to be A/D converted, and D is a digital value of V.
As described, the A/D converter that converts an analog input voltage to a digital value generally has an offset error, a full scale error, and a nonlinearity error due to the characteristics of a circuit in the A/D converter. This causes conversion errors as shown in FIG. 13 between a voltage (V′) calculated as an analog input voltage based on the ideal A/D conversion characteristics shown in FIG. 13 using the digital value (D), which is obtained by A/D converting the actual analog input voltage (V), and the voltage (V) calculated as an analog input voltage based on the actual A/D conversion characteristics shown in FIG. 13 using the digital value (D).
When the reference voltage (Vref) of the A/D converter is contained in the equation, high accuracy is required for Vref as well. Some high-accuracy A/D converters have functions of reducing these conversion errors generated inside the converter, and of being capable of adjusting the conversion errors. However, such A/D converters are generally expensive due to complex circuitry, and also suffer from complex control.
To address this, a method has been proposed that corrects a digital value of another analog input voltage based on a digital value obtained by A/D converting a known analog input voltage, in the configuration as shown in FIG. 12. According to the conventional technology disclosed in Japanese Patent Application Laid-open No. 2005-26830, based on a digital value (D) obtained by A/D converting a known high-accuracy analog input voltage (V), a converted value (V1′) corresponding to an analog input voltage (V1) is calculated using a digital value (D1) that is obtained by AD converting an unknown analog input voltage (V1).
This calculation method is expressed by Equation 2:V1′=V/D×D1
A/D conversion characteristics calculated by using Equation 2 are indicated as “A/D conversion characteristics of related art” in FIG. 13. With this method, a converted value corresponding to an analog input voltage can be calculated, without being affected by the fluctuation in Vref.
As described, an image forming apparatus needs to calculate a high-accuracy analog input voltage to control the power of the fixing heater to be suitable for an analog input voltage that varies depending on the environment of a connected external power source (e.g., the input voltage drop caused by using a private power generator or other power sources, or by connecting the image forming apparatus to other devices via a plurality of lines).
A typical A/D converter as disclosed in Japanese Patent Application Laid-open No. 2005-26830 corrects a digital value of an analog input voltage, based on a digital value of an analog input voltage with higher accuracy than an A/D conversion reference voltage. Because the correction is based on a single analog input voltage with higher accuracy than the A/D conversion reference voltage and its digital value, a converted value calculated by using Equation 2 fluctuates with respect to values obtained by A/D converting analog input voltages in a wide range. This poses a problem with accuracy of A/D converter, due to the lack of consideration on the nonlinearity error in the A/D converter, which will be described in detail later.
Specifically, the A/D conversion characteristics calculated by using Equation 2 (the A/D conversion characteristics of related art shown in FIG. 13) indicate the lack of consideration on the conversion errors in the A/D converter, particularly on the nonlinearity error. Thus, depending on the settings on a known analog input voltage (V), the conversion errors are further increased as an unknown analog input voltage (V1) to be A/D converted is closer to Vref. This may result in the analog input voltage (V1) exceeding the full scale error.